1. Field of the Invention
The present invention relates to a device and a method for driving a light-emitting display panel in which a large number of light-emitting elements which exhibit different emission colors as display pixels to perform full-color display or multi-color display.
2. Description of the Related Art
Along with the popularization of a mobile telephone, a personal digital assistant (PDA), and the like, a demand for a display panel which has a high-definition image display function and can realize a small thickness and a low power consumption increases. As a display panel which satisfies the demand, liquid crystal panels are conventionally applied to a large number of products. On the other hand, in recent years, an organic EL (Electro-Luminescence) element which takes advantage of characteristics of a self-emitting display element is practically used. The display panel draws attention as a next-generation display panel which is replaced with a conventional liquid crystal display panel. This is caused by a background in which an organic compound which can expect preferable light-emitting characteristics is used in a light-emitting function layer of an element to achieve practical high efficiency and practical long life.
The organic EL element, for example, is basically formed such that a transparent electrode consisting of, e.g., ITO, a light-emitting function layer consisting of an organic material, and a metal electrode are sequentially stacked on a transparent substrate such as a glass substrate. The light-emitting function layer may be a single layer consisting of an organic light-emitting layer, a two-layer structure consisting of an organic hole transportation layer and an organic light-emitting layer, a three-layer structure consisting of an organic hole transportation layer, an organic light-emitting layer, and an organic electron transportation layer, or a multi-layer structure obtained by inserting an electron or hole-implanted layer between these appropriate layers.
The organic EL element can be electrically expressed by an equivalent circuit as shown in FIG. 1. More specifically, the organic EL element can be electrically replaced with a configuration constituted by a diode component E serving as a light-emitting element and a parasitic capacitive component Cp coupled in parallel to the diode component E. The organic EL element is considered as a capacitive light-emitting element.
When a light-emitting drive voltage is applied to the organic EL element, first, electric charges corresponding to the electric capacitance of the element flow into the electrode as a displacement current and are accumulated in the electrode. Subsequently, when the voltage exceeds a predetermined voltage (light-emitting threshold voltage=Vth) inherent in the element, a current begins to flow from one electrode (anode side of the diode component E) to the organic layer constituting the light-emitting layer. It can be understood that light emission occurs with an intensity which is in proportion to the current.
FIGS. 2A to 2D show light-emitting static characteristics of such an organic EL element. According to this, the organic EL element, as shown in FIG. 2A, emission occurs with a luminance L which is appropriately proportional to a drive current I. As indicated by a solid line in FIG. 2B, a drive voltage V is equal to or higher than an emission threshold voltage Vth, the current I rapidly flows to emit light.
In other words, when the drive voltage is equal to or lower than the emission threshold voltage Vth, a current rarely flows in the EL element, and the EL element does not emit light. Therefore, the EL element has the following luminance characteristic. That is, as indicated by a solid line in FIG. 2C, in an emittable region in which the drive voltage is larger than the threshold voltage Vth, as the voltage V applied to the EL element increases, an emission luminance L increases.
On the other hand, it is known that the organic EL element has physical properties which change in long-termuse to increase a forward voltage Vf. For this reason, In the EL element, as shown in FIG. 2B, a V-I (L) characteristic changes in a direction indicated by an arrow (characteristic indicated by a brokenline) depending on actual operating time. Therefore, the luminance characteristic also decreases.
Furthermore, it is known that the luminance characteristic generally changes as indicated by a broken line in FIG. 2C depending on a temperature. More specifically, the EL element has the following characteristics. That is, in an emittable region in which the drive voltage is larger than the emission threshold voltage, as the voltage V applied to the EL element increases, the emission luminance L of the EL element increases. However, the temperature increases, the emission threshold voltage decreases. Therefore, a minimum applied voltage with which the EL element is set in an emittable state decreases as the temperature increases. Even though a predetermined emittable applied voltage is given, the EL element is bright at a high temperature and dark at a low temperature. That is, the luminance is dependent on temperature.
In addition, the EL elements disadvantageously have luminous efficiencies to a drive voltage which change depending on emission colors. As the luminous efficiencies of EL elements which can be practically used and emit R (Red), G (Green), and B (Blue) lights, in an early stage, as generally shown in FIG. 2D, the emission efficiency of G is high, and the emission efficiency of B is the lowest. Each of the EL elements which emit R, G, and B lights has an aging characteristic and a temperature dependence as shown in FIGS. 2B and 2C.
Therefore, when EL elements which emit R, G, and B lights are arranged to try to perform, e.g., full-color display, a color balance is disrupted due to a change in environment temperature or aging, and display quality cannot be easily held at a predetermined level. In particular, in a drive device for an active matrix display panel having a configuration in which EL elements are driven at a constant voltage by switching operations of TFTs, as indicated by V-I (L) characteristics shown in FIGS. 2A to 2D, an emission luminance largely varies with a variation of the forward voltage Vf of each element to pose a problem of considerable deterioration of display quality.
For this reason, in order to solve the above problem, monitor elements which monitor the forward voltages Vf of the EL elements which emit R, G, and B lights are prepared. A device for driving a light-emitting display panel in which drive voltages applied to the EL elements which emits the color lights are independently controlled based on the forward voltages Vf obtained by the monitor elements is disclosed in Japanese Unexamined Patent Publication No. 2003-162255.
As described above, when the display device designed to independently control drive voltages applied to display EL elements which emit R, G, and B lights is employed in, e.g., a mobile device, a battery voltage serving as a primary power supply is boosted to be given to the display EL elements of the respectively colors.
In this case, as means which boosts the battery voltage serving as the primary power source, a DC-DC converter constituted by a switching regulator is generally used. When the DC-DC converter is used, an operation which uses forward voltages Vf obtained by monitor elements corresponding to R, G, and B as control voltages and boosts drive voltages applied to display EL elements based on the control voltages is executed. For this reason, even though the forward voltages change due to aging or temperature dependence of the elements, the relationship among optimum drive voltages well-balanced with respect to R, G, and B can be maintained.
On the other hand, in the configuration, when the control voltages of the DC-DC converter increase under any fault, or when the circuit which supplies the control voltage is set in an open state due to any fault, an output voltage from the DC-DC converter considerably increases to damage not only the pixels arranged on the display panel but also driver circuits or the like which luminescently control the pixels.
Therefore, when the boosting DC-DC converter is used, a voltage limiter which suppresses the output voltage of the converter from excessively increasing due to an unexpected situation must be simultaneously used. When the voltage limiter is used together with the converter as described above, a configuration in which zener diodes functioning as voltage limiters are connected to control voltage input terminals of the DC-DC converters corresponding to R, G, and B, respectively can be preferably employed.
As has been described above, the forward voltages of the monitor elements corresponding to R, G, and B gradually increase due to aging. Therefore, as described above, in the DC-DC converters which are used together with the voltage limiters, when the forward voltage of any one of the monitor elements corresponding to R, G, and B reaches a forward voltage at which the corresponding voltage limiter operates, a drive voltage corresponding to the monitor element is regulated, and the relationship among the optimum drive voltages corresponding to the characteristics of R, G, and B cannot be maintained.
Subsequently, because the degree of aging further advances, it is more and more impossible to obtain the relationship among optimum drive voltages. Therefore, a color balance (white balance) is kept disrupted. Recovery of the color balance is difficult.
FIG. 3 is to explain the above situation. Reference symbols R, G, and B in FIG. 3 indicate characteristics of the monitor elements corresponding to the respective colors described above. An abscissa (T) indicates elapsed time, and an ordinate (V) indicates a forward voltage of the monitor element. In the example shown in FIG. 3, for example, aging of the monitor element corresponding to B advances, the forward voltage of the monitor element is first to reach a forward voltage Vf1 at which the voltage limiter operates. Elapsed time until this is represented by T1 for descriptive convenience.
More specifically, until the time T1, the output voltages of the converters are controlled by the forward voltages of the monitor elements corresponding to R, G, and B, respectively. For this reason, the color balance (white balance) can be maintained. However, after the time T1, the output voltage from the converter corresponding to B is limited to a limiter operation. For this reason, the color balance on the display panel is disrupted.
In the example shown in FIG. 3, the limiter levels corresponding to R, G, and B are equal to each other. Therefore, when time has further elapsed, the forward voltage of the monitor element corresponding to R secondly reaches the voltage Vf1 to operate the limiter. Finally, the forward voltage of the monitor element corresponding to G reaches the voltage Vf1 to operate the limiter.
In this manner, in the situation in which the limiter functions corresponding to R, G, and B operate, the drive voltages applied to the display elements are approximately equal to each other. For this reason, the color balance (white balance) is kept disrupted due to difference of light-emitting efficiencies of R, G, and B. The color balance cannot be recovered.